Purification and enzymatic characterization of the hepatitis B virus ribonuclease H, a new target for antiviral inhibitors

- Appending an MBP tag to the HBV RNaseH domain allows it accumulate to high levels in E. coli.
- Appending a HisX6 tag to the RNaseH allows single-stage purification of monomeric soluble protein to homogeneity.
- The HBV RNaseH requires an unbulged RNA:DNA heteroduplex of ≥14 nt for cleavage.
- The enzyme has no strong positional or sequence specificity for NRA cleavage.
- The HBV RNaseH has both endoribonucleolytic and 3′-5′ exoribonucleolytic activities.

Hepatitis B virus (HBV) reverse transcription requires coordinated function of the reverse transcriptase and ribonuclease H (RNaseH) activities of the viral polymerase protein. The reverse transcriptase has been biochemically characterized, but technical difficulties have prevented both assessment of the RNaseH and development of high throughput inhibitor screens against the RNaseH. Expressing the HBV RNaseH domain with both maltose binding protein and hexahistidine tags led to stable, high-level accumulation of the RNaseH in bacteria. Nickel-affinity purification in the presence of Mg2+ and ATP removed co-purifying bacterial chaperones and yielded nearly pure monomeric recombinant enzyme. The endonucleolytic RNaseH activity required an DNA:RNA duplex ≥14 nt, could not tolerate a stem-loop in either the RNA or DNA strands, and could tolerate a nick in the DNA strand but not a gap. The RNaseH had no obvious sequence specificity or positional dependence within the RNA, and it cut the RNA at multiple positions even within the minimal 14 nt duplex. The RNaseH also possesses a processive 3′-5′ exoribonuclease activity that is slower than the endonucleolytic reaction. These results are consistent with the HBV reverse transcription mechanism that features an initial endoribonucleolytic cut, 3′-5′ degradation of RNA, and a sequence-independent terminal RNA cleavage. These data provide support for ongoing anti-RNaseH drug discovery efforts.